1. Field of the Invention
The present invention relates to an antenna suitable for use in radio communication of a wireless starting system or the like for a vehicle.
2. Description of Related Art
Conventionally, an antenna employing a monopole antenna has been used for performing radio communication of a wireless starting system or the like for a vehicle. However, since such monopole antenna as a whole tends to be long in one direction, there has been provided a small-sized inverted-L antenna manufactured by bending the monopole antenna.
In such inverted-L antenna, a capacitive reactance component is generated between a ground surface of a circuit board and a horizontal section of an antenna element parallel with the ground surface. When the height of the horizontal section of such inverted-L antenna is low, the impedance of the antenna decreases, thereby causing the impedance difference to be matched to increase, thus making it difficult to perform impedance matching with respect to a feeder of 50Ω. Also, when the impedance difference to be matched becomes large, the loss on a matching circuit inserted between the feeder and the antenna element increases, thus impairing the efficiency of the antenna.
As described in Laid-open Japanese patent publication No. Hei 07-288415 and “small-sized antenna and system application” (page 40, page 41), by K-Laboratory publishing, there has been provided an inverted-F antenna aimed to facilitate impedance matching between an antenna element and the feeder of 50Ω. In such inverted-F antenna, a horizontal section is connected to a vertical section rising from a ground surface, and a vertical stub is provided in the vicinity of the vertical section, such that electricity is supplied to an end portion of the vertical stub. In this way, the effect of capacitance due to the reactance component is cancelled, thereby making it easy to perform impedance matching with respect to the feeder of 50Ω and reducing the loss generated on the matching circuit.